The present invention relates to an antenna device for an automobile for suppressing generation of noises.
As an antenna device for receiving radio waves for an automobile, there has been known such ones having antenna conductors for AM broadcasts, FM broadcasts or AM/FM broadcasts which are provided on the surface or inside the rear windowglass of the automobiles instead of rod antenna. The antennas provided on or in the rear window glass of the automobiles, having no part projecting from the car body unlike the rod antenna or the lid antenna, have many advantages that they are harmless to persons, are not easily broken, do not become rusty, hence there is no change of performance, and provide excellent appearance for the automobiles.
In such type of antenna devices, it is possible to obtain desired performance, i.e. increasing a gain of antenna by suitably designing the pattern of an antenna conductor or antenna conductors provided on or in a glass plate. However, the surface area of the glass plate is not sufficiently large when the antenna device is installed in a window glass for an automobile. Accordingly, the distance between the antenna conductors and the body of an automobile can not be sufficiently large, whereby a leak current from the antenna conductor is large and a gain of antenna becomes insufficient.
In case of the antenna device to be installed in or on the rear window glass of an automobile, an electric heating type defogger for preventing the rear window glass from being cloudy is provided together with the antenna conductor formed in a predetermined pattern. In particular, when the antenna conductor is placed reclose to a heater, a current in the antenna conductor leaks to the body of the automobile through the power feeding part of the electric heating type defogger thereby resulting in a loss of the gain. In order to eliminate the loss of gain, there has been proposed to attach a choke coil to a suitable position of a power feeding point of the electric heating type defogger. However, there is a tendency of reducing the antenna gain because the choke coil has not a sufficient high frequency current preventing function when the choke coil is used for an antenna device for an automobile.
In a conventional antenna device, there was practiced to insert a pre-amplifier 331 at a suitable position of a power feeding line between the power feeding terminal 311 of antenna conductors 306 and a radio wave receiver 312 in order to compensate a loss of antenna gain as shown in FIG. 22. However, there occurred cross modulation in a strong electric field due to the presence of the pre-amplifier to thereby increase a noise level. Further, since it was necessary to connect the pre amplifier in addition to the radio wave receiver in the above-mentioned system, the manufacturing cost is increased. Furthermore, the pre-amplifier to be disposed near the antenna device fairly restricts the condition of designing an automobile, e.g. in assuring a space for the pre-amplifier. Accordingly, it has been expected to develop an antenna device for an automobile capable of obtaining a high gain and reducing noises without the necessity of the pre-amplifier.